Merge branch 'ta/config-set-2'

Merge branch 'ta/config-set-2'

Update git_config() users with callback functions for a very narrow
scope with calls to config-set API that lets us query a single
variable.

* ta/config-set-2:
  builtin/apply.c: replace `git_config()` with `git_config_get_string_const()`
  merge-recursive.c: replace `git_config()` with `git_config_get_int()`
  ll-merge.c: refactor `read_merge_config()` to use `git_config_string()`
  fast-import.c: replace `git_config()` with `git_config_get_*()` family
  branch.c: replace `git_config()` with `git_config_get_string()
  alias.c: replace `git_config()` with `git_config_get_string()`
  imap-send.c: replace `git_config()` with `git_config_get_*()` family
  pager.c: replace `git_config()` with `git_config_get_value()`
  builtin/gc.c: replace `git_config()` with `git_config_get_*()` family
  rerere.c: replace `git_config()` with `git_config_get_*()` family
  fetchpack.c: replace `git_config()` with `git_config_get_*()` family
  archive.c: replace `git_config()` with `git_config_get_bool()` family
  read-cache.c: replace `git_config()` with `git_config_get_*()` family
  http-backend.c: replace `git_config()` with `git_config_get_bool()` family
  daemon.c: replace `git_config()` with `git_config_get_bool()` family